Although the mating surfaces of pairs of surfaces to be sealed, such as mating engine head and block surfaces, appear to be smooth and flat, most frequently they are not sufficiently smooth to provide an effective seal. Accordingly, a gasket is required to be placed between them to provide an effective seal. That is uniformly true for head gaskets for internal combustion engines.
There are a wide variety of head gasket designs which include inter alia, metal gaskets, gaskets of fibrous materials, sandwich type gaskets which combine a metallic core and compressible fiber-elastomer facing materials laminated to the core, and so forth. Some such gaskets incorporate specially armored combustion openings, thereby to assist in withstanding the pressure and heat generated in the combustion chambers of the engines with which they are used, while also displaying sufficient resiliency and sealing strength to effect the necessary seal.
An important physical property of any gasket is good torque retention and acceptable levels of compression set. In general, torque retention is associated with the amount of compressible material in a gasket. Compression set is defined as the residual deformation of a material after the removal of the force tending to compress it together. The compressibility of gaskets which employ compressible facing materials depends upon the type of compressible gasket facing material employed. In any event, the facing material must be sufficiently compressible so that it will conform to variations in the surfaces to be sealed, and so that it will be adequate to seal despite variations in the torquing load exhibited along the mating surfaces to be sealed.
It has been determined that additional sealing elements may be applied at selected locations to gaskets to provide gaskets of enhanced sealing capability. Thus, deformable beads and bead segments have been applied to gaskets to act as seals and to cooperate with the entire structure to enhance the sealing capacity of the facings. Such beads and bead segments deform under pressure or load and aid in realizing fluid-type seals between the facing parts to be sealed. Selective locations of the sealant beads or segments allow for improved sealing by giving high unit loadings where required. Such beads also permit the use of thinner compressible substrates or facings, and therefore contribute to reducing torque loss.
Where such beads comprise elastic sealant material, that permits selective contact of the gasket assembly with the surfaces to be sealed. In turn this permits higher clamping pressures where desired. Less bolting is required under such circumstances and, therefore, lighter weight and strength gasket components may be used.
For a number of years now, sealing components, such as elastomeric sealants in the form of beads, have been applied to gasket assemblies by a silk-screening process. An early patent disclosing and describing such a process is Hillier U.S. Pat. No. 3,477,867. In such a process, a selected gasket assembly or substrate is positioned adjacent a suitably patterned silk screen, and a liquid, paste-like sealant is squeezed through the screen onto the substrate. The sealant is suitably cured and the gasket is then ready for use.
Perhaps the most widely used sealants in silk-screening processes for gasket assemblies have been room temperature vulcanizable silicone rubbers (RTV silicones). RTV silicones have a number of advantages including relatively high temperature resistance, good strength under load, relative ease of application, among others. However, such materials also have drawbacks in terms of curing time (they must be cured at elevated temperatures because room temperature curing requires too long a holding time for practical mass production usage), sensitivity of the material to certain substrates which inhibit curing, and the like.
With current, two-component silk-screenable RTV silicone compositions and typical fiber-elastomer gasket facings, it has been found that barrier layers must be applied to the facings prior to silk-screening of the silicone to prevent "poisoning" of the silicone catalyst. Typically, such 2-part RTV silicones use a platinum catalyst to produce an addition-type cure at temperatures of about 375.degree. F. for 2 to 3 minutes. When gasket facings have sulfur, nitrogen or halogen materials (typically found in nitrile rubber, neoprene curing agents, and protective agents, such as anti-oxidants, used in rubbers), curing is inhibited and adhesion of the sealant to the substrate is therefore diminished. In severe cases, the sealant will not cure at the interface, and adhesion to the substrate is therefore minimal. Of course, the longer the curing time at elevated temperatures, such as at 375.degree. F., the more damage is done to the facing layers which tend to degrade at such temperatures.
To avoid all of this, barrier coats, as of phenolic materials, are applied to the facing layers prior to application of the sealant. Frequently, two such coats are necessary, each requiring an extended "cure" at an elevated temperature, such as at 375.degree. F. for eleven minutes. This also tends to degrade many facings, causing them to harden and to become more brittle, resulting in gaskets of diminished sealing capability, because, for example, the harder the facing layer, the more torque is required to effect a seal in the zone in which the facing layers are to effect a seal. For example, nitrile rubbers tend to degrade at temperatures above 275.degree. F.
Further, when high temperature cures of the RTV silicone and of the barrier coats are used and the facings become hard and brittle, not only does the gasket tend to become less effective as a seal, but the facings tend to impress less easily, causing overlying silicone beads to deform and squeeze laterally more than intended, tending toward promoting destruction of an effective bead seal. As such, the silicone used must be formulated to accommodate to this and such formulations frequently have a greater compression set (less recovery) than would be desireable.
Thus, it would be highly desireable to avoid the need for barrier coats, and the problems and expense which inhere in their use, to provide an improved silk-screening process for gaskets, to provide materials for silk-screening process which would avoid these and other problems and drawbacks to which users of conventional silk-screening processes must now accommodate and adapt, and to produce silk-screened gaskets in a relatively simple and straightforward manner.